Orientation sensing devices typically use IMUs containing three types of sensors, namely, gyroscopes, accelerometers, and magnetometers, to determine their orientation relative to Earth. A method for performing such operation is disclosed in the 14/873,946 application. For example, the IMU's accelerometers and magnetometers can be used to determine a field orientation estimate of the orientation sensing device based upon Earth's gravitational and magnetic fields when the device is relatively still. A separate gyroscopic orientation estimate can be also determined using the previous device orientation estimate and the most recent gyroscope reading. The field and gyroscopic orientation estimates can be combined based on relative sensor error estimates to provide an output device orientation estimate.
The IMU's accelerometers measure the orientation sensing device's centripetal acceleration in addition to gravitational acceleration. Thus, when the orientation sensing device is undergoing significant centripetal acceleration relative to gravitational acceleration, the field orientation estimate generation is adversely affected, resulting in a poor orientation estimate, which is discarded because of its poor value.
When the orientation sensing device is relatively still and its orientation is not changing, the current field orientation estimate produces the same result as the previous estimate, due to the lack of changing orientation. Even after the device has moved and is still again, the gyroscopic orientation estimate may be as accurate as the field orientation estimate, due to the inherent jitter in the IMU's magnetometer readings.
Methods exist for fusing IMU sensor values together to determine device orientation and mitigate sensor offsets and jitters, but they typically employ Kalman filters, which require all of the sensor inputs (gyroscope, accelerometer, and magnetometer) all of the time. As a result, the IMUs employing these methodologies typically require more expensive and current consuming processors. The larger current consumption results in the need for larger batteries or reduced operational time.